The present invention relates to an image forming apparatus and image forming method based on electrophotographic technology.
In an image forming apparatus based on electrophotographic technology, a developer bearing member for bearing the developer (toner in the case of a one-component development, and toner and carrier in the case of a two-component development) is moved relative to the photoreceptor with an electrostatic latent image formed thereon, whereby the electrostatic latent image on the photoreceptor is developed. In this case, to ensure that the background fog (toner adhered to the background where toner should not adhere) does not occur, a potential difference is provided between the surface potential of the photoreceptor background portion and the bias potential of the developer bearing member (hereinafter referred to simply as “development bias” in some cases).
However, even if a proper potential difference is provided between the surface potential of the photoreceptor background portion and the bias potential of the developer bearing member, the characteristics of the developer such as the amount of charged toner and quantity of the developer are changed by a change with the passage of time due to large number of printing, environmental condition change and long period of time to be left, with the result that a background fog (hereinafter referred to simply as “fog” in some cases) occurs.
One of the efforts to solve this problem is disclosed in the Patent Document 1 (Japanese Non-Examined Patent Publication: Tokkaihei 5-224512) wherein toner density of toner fog is detected by a toner sensor while the development bias is changed, and the characteristic curve of toner density with respect to development bias is obtained. If the development bias capable of outputting the toner density when toner is no adhered is higher than a reference level, copying operation is carried out by increasing development bias by a predetermined amount, thereby solving the problem caused by a rise in fogging level.
However, the amount of toner adhered due to fog is very small. The conventional optical sensor described in the Patent Document 1 and others has been characterized by poor detection accuracy and poor reliability. This has been the problem yet to be solved in the conventional method.